Go to the Reinstall Lion link in my previous post for instructions. You shouldn't lose anything. If you follow's Artie's idea of reinstalling completely from Time Machine, you probably will lose some emails because TM will revert to whatever you had on that date.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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